If you're using an absolute URL, just pass it in as the first argument:
// Don't worry, this is just the website for the musician Phoebe Bridgers.
// It's one of my favourite URLs.
const url = new URL("https://phoebefuckingbridgers.com");
If you're using a relative URL, you need to pass in a base URL as the second argument:
const url = new URL("contact", "https://phoebefuckingbridgers.com");
URL interface is supported in all modern browsers, but not Internet Explorer (IE). If you need to support IE, there is a polyfill you can use. The easiest way is to use the default bundle from polyfill.io, which includes it.